The basic principles of x‐ray image formation and interpretation in radiography have remained essentially unchanged since Röntgen first discovered x rays over a hundred years ago. The conventional approach relies on x‐ray absorption as the sole source of contrast and draws exclusively on ray or geometrical optics to describe and interpret image formation. This approach ignores another, potentially more useful source of contrast—phase information. Phase‐sensitive techniques, which can be understood using wave optics rather than ray optics, offer ways to augment or complement standard absorption contrast by incorporating phase information.

New approaches that can detect x‐ray phase shifts within soft tissues show promise for clinical and biological applications.

Copenhagen, a thought‐provoking drama by Michael Frayn that has played to sold‐out audiences in London, has now opened in New York with much well‐deserved attention. Despite the sparse set, the three‐person cast, and the technical nature of the subject matter, this play appears to speak on some level to almost everyone. (See PHYSICS TODAY, May, page 51.)

What was Werner Heisenberg trying to tell Niels Bohr during his visit to Copenhagen in 1941, and what did he want from Bohr?

During World War II, the German atomic scientists were trying to produce electric power by creating a chain reaction in uranium. Werner Heisenberg hoped that this achievement would impress the allies if the war was lost.

The Farm Hall tapes show that Werner Heisenberg did not know how to calculate the critical mass in 1945, indicating that he did not work on atomic bombs during the war.

Werner Heisenberg is suddenly in the news again, this time thanks to the award‐winning new play Copenhagen by Michael Frayn. The play centers on the ambiguous reasons for Heisenberg's visit in 1941 to his early mentor, Niels Bohr, in German‐occupied Copenhagen. It speculates on what might have transpired during the evening walk they took at that time, which Bohr ended abruptly, disturbed by something Heisenberg had said. (See David Cassidy's article on page 28.)

Albert Einstein was the early model and inspiration for Heisenberg, but scientific conflicts and political stresses marred their relationship.

In 1998 two related but independent groups sent balloon‐borne microwavetelescopes aloft to study fluctuations in the cosmic microwave background(CMB) at fine angular resolution. In August of that year, the Maxima telescope spent one night at 40 km above Texas. And at the end of the year, its “sister” telescope, called Boomerang, took advantage of the steady circumpolar winds of the austral summer to complete a 10‐day stratospheric circumnavigation of Antarctica.

The power spectrum of the microwave background's tiny point‐to‐point temperature fluctuations is a superb probe of cosmic curvature.

Researchers have been pushing atoms around with laser beams for many years, cooling them to record low temperatures and capturing them in confined spaces. Most such efforts require many photons. The field of a single photon is not generally strong enough to corral an atom—unless, that is, the light field is enhanced by confinement within a tiny cavity and the atom enters at a crawl. Then the coupling energy of the atom to the field can exceed the atom's kinetic energy, and the atom can become ensnared. A group from Caltech and the University of Auckland, and also a group at the Max Planck Institute for Quantum Optics in Garching, Germany, recently created the required conditions by combining cavity quantum electrodynamics with laser trapping and cooling; both groups succeeded in confining single atoms within optical cavities with an average of one photon in the cavity.

The field of a single photon can not only trap atoms but also signal its position.

The precision with which we know a fundamental constant usually gets better with the passing years. Just the opposite, however, was happening recently to G, Newton's gravitational constant. In 1987, CODATA, the international arbiter of metrology, was quoting an uncertainty of parts per million for G, the most poorly known of the fundamental constants. But then things got even worse. Several recent measurements have given values many standard deviations outside these limits, forcing CODATA to increase its assigned uncertainty to ppm for the latest compilation.